According to JP-A-2003-336558 (U.S. Pat. No. 6,767,179 B2), a fuel pump includes an impeller having vane grooves in the outer circumferential periphery of the impeller. The impeller rotates in the fuel pump, so that the fuel pump pumps fuel. Specifically, as shown in FIGS. 6A, 6B, an impeller 300 has end surfaces with respect to the direction of the rotation axis thereof. The end surfaces of the impeller 300 respectively have vane grooves 302, 304. The impeller 300 has communication holes 306 on the side of the inner circumferential periphery of the vane grooves 302, 304. The communication holes 306 axially penetrate the impeller 300. Pump passages are formed on both sides of the impeller 300 with respect to the axial direction of the impeller 300.
The pump passages respectively extend along the vane grooves 302, 304. When the impeller 300 rotates, fuel is respectively pressurized in the pump passages on both sides relative to the rotative direction. Fuel, which is in the pump passage on one axial side, passes through the communication holes of the impeller, so that the fuel flows together with fuel in the pump passage on the other axial side, thereby being discharged from the outlet of the pump passage.
In this structure, when the impeller 300 rotates, and fuel is pressurized in the pump passages using the vane grooves 302, 304, pressure fluctuation arises in fuel at a frequency υ that is calculated by the following formula.υ=(number of the vane grooves)×(rotation speed of the impeller).
As shown in FIG. 7, noise arises at a frequency the corresponding to the pressure fluctuation. The vane grooves 302, 304, which are formed on both sides of the axial end surfaces of the impeller 300, are displaced from each other for a half pitch thereof along the rotative direction. Therefore, in FIG. 7, the distribution of the noise has two peaks. Specifically, another peak arises in the distribution of the noise at particular frequency, which is twice as the other frequency.